1. Field of the Invention
The present invention relates to a longitudinally coupled resonator type surface acoustic wave filter, and more particularly, to a longitudinally coupled resonator type surface acoustic wave filter, in which at least three IDTs are arranged in a surface wave propagating direction.
2. Description of the Related Art
Recently, with the increasing numbers of customers and the diversifying services, many more mobile phone systems use transmission-side frequency bands and reception-side frequency bands close to each other. In addition, to prevent interference with other communication systems, there is a demand for increased attenuation very close to a pass band.
As a result, there is a strong demand for obtaining an attenuation band very closely to the pass band in surface acoustic wave filters widely used as band pass filters for an RF stage in a mobile phone.
On the other hand, recently, to reduce the number of components, there has been a growing demand for a balance-unbalance conversion function, the so-called balun function. Thus, as the band pass filter for an RF stage in a mobile phone, longitudinally coupled resonator type surface acoustic wave filters easily adaptable to the balance-unbalance conversion function have been used. Various types of longitudinally coupled resonator type surface acoustic wave filters (for example, Japanese Unexamined Patent Application Publication No. 5-267990, etc.) have been provided.
However, in the longitudinally coupled resonator type surface acoustic wave filter, an undesired response referred to as a transversal response appears on a frequency side higher than a pass band. Thus, for example, it is impossible to obtain large attenuation on the pass band high frequency side, which is required in a mobile phone system of the PCS system.
FIG. 10 shows a schematic plan view illustrating the electrode structure of a longitudinally coupled resonator type surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 10-190394. In the longitudinally coupled resonator type surface acoustic wave filter 101, IDTs 102 to 104 are arranged in a surface wave propagating direction. On both sides of the IDTs 102 to 104 reflectors 105 and 106 are provided. In this case, an IDT gap P between the IDTs 102 and 103 is differentiated from an IDT gap Q between the IDTs 103 and 104. With this arrangement, the above transversal response is suppressed. In the conventional longitudinally coupled resonator type surface acoustic wave filter of the 3-IDT type, in addition to a zero-order mode and a second mode, a resonance mode (hereinafter referred to as a resonance mode generated by the IDT to IDT gap) is generated at a peak in the strength distribution of the surface acoustic wave in the gaps between the IDTs. These three resonance modes are used to produce the pass band of the filter.
In the technology described in the conventional art, by differentiating the IDT gap P from the IDT gap Q the resonance mode caused by the IDT to IDT gap is generated in each of the gaps between the IDTs. The two resonance modes are used to produce the pass band. In this technology, attenuation generated near the pass band, particularly, on the high frequency side thereof, is reduced to a greater extent than in the conventional longitudinally coupled resonator type surface acoustic wave filter.
However, the IDT gaps P and Q must be substantially differentiated from 0.50 times the wavelength determined by electrode-finger pitches. As a result, since the propagation continuity of a surface acoustic wave is lost, insertion loss in the pass band is greatly deteriorated.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a longitudinally coupled resonator type surface acoustic wave filter which eliminates the transversal response, and thereby obtains sufficient attenuation on a pass band high frequency side, and which greatly reduces insertion loss in the pass band.
A longitudinally coupled resonator type surface acoustic wave filter according to preferred embodiments of the present invention includes a piezoelectric substrate and first, second and third IDTs each having a plurality of electrode fingers, provided on the piezoelectric substrate and arranged in a surface wave propagating direction such that the second IDT is interposed between the first and the third IDTs.
The first and second IDTs have narrow electrode-finger pitch sections which have an electrode-finger pitch that is narrower than the remaining electrode-finger pitches, at respective end portions of the first and second IDTs that are adjacent to each other. The second and third IDTs have narrow electrode-finger pitch sections that have an electrode-finger pitch that is narrower than the remaining electrode-finger pitches, at respective end portions of the second and third IDTs, which are adjacent to each other.
According to the first preferred embodiment of the present invention, the electrode-finger pitch of the narrow electrode-finger pitch sections in the first and second IDTs is different from the electrode-finger pitch of the narrow electrode-finger pitch sections in the second and third IDTs.
According to the second preferred embodiment of the present invention, the number of the electrode fingers of the narrow electrode-finger pitch sections in the first and second IDTs is different from the number of the electrode fingers of the narrow electrode-finger pitch sections in the second and third IDTs.
According to the third preferred embodiment of the present invention, only the first and second IDTs or only the second and third IDTs have narrow electrode-finger pitch sections which have an electrode-finger pitch narrower than the remaining electrode-finger pitches, at respective end portions of the first and second IDTs adjacent to each other, or at the respective end portions of the second and third IDTs adjacent to each other.
According to the unique arrangements of these preferred embodiments, transversal response is greatly minimized. In addition, by adjusting the pitches of the narrow electrode-finger pitch sections, without increasing the insertion loss of the pass band, the pass-band width is easily adjusted. It is possible to combine first and second preferred embodiments to further reduce the transversal response.
The first to third preferred embodiments of the present invention are successfully applied to a longitudinally coupled resonator type surface acoustic wave filter including first and second longitudinally coupled resonator type surface acoustic wave filter units. More specifically, the pitches and/or the numbers of electrode fingers of the narrow pitch electrode finger sections are different between the first and second longitudinally coupled resonator type surface acoustic wave filter units. Alternatively, the narrow pitch electrode finger sections may be provided only in either first or second longitudinally coupled resonator type surface acoustic wave filter units. With the above-described configurations, the bandwidth is easily adjusted without degrading the insertion loss.
In the longitudinally coupled resonator type surface acoustic wave filter according to each preferred embodiment of the present invention, when the second IDT has an even number of electrode fingers or when each of the second and fifth IDTs has even numbers of electrode fingers, the difference of the electrode-finger pitches between the narrow electrode-finger pitch sections on both sides of the central IDT can be reduced. Thus, as compared with the case in which the central IDT has odd electrode fingers, the narrowest pitch between the electrode fingers is widened. As a result, the longitudinally coupled resonator type surface acoustic wave filter of each of the first and second preferred embodiments of the present invention are easily produced in a stable manner. Furthermore, as shown in the aforementioned third preferred embodiment, the insertion loss in the pass band is further reduced.
When at least one IDT is subjected to thinning out and weighting, particularly, when the thinning-out and weighting are asymmetrical to the propagating-direction center of the longitudinally coupled resonator type surface acoustic wave filter on both sides of a surface wave propagating direction, the transversal response is more effectively suppressed.
When the longitudinally coupled resonator type surface acoustic wave filter is connected in series and/or in parallel to the surface acoustic wave resonator, attenuation outside a pass band is greatly increased. Furthermore, in this case, when at least one IDT of the longitudinally coupled resonator type surface acoustic wave filter is subjected to weighting, a transversal response is more effectively suppressed. As a result, since the number of the surface acoustic wave resonators is reduced, miniaturization of the element size is achieved.
In the longitudinally coupled resonator type surface acoustic wave filter according to preferred embodiments of the present invention, when a balancing input terminal or a balancing output terminal is provided and an unbalancing output terminal or an unbalancing input terminal is provided, according to preferred embodiments of the present invention, the longitudinally coupled resonator type surface acoustic wave filter has a balancing/unbalancing conversion function and effectively eliminates a transversal response.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.
FIG. 1 shows a schematic plan view of a longitudinally coupled resonator type surface acoustic wave filter according to a first preferred embodiment of the present invention.
FIG. 2 shows a graph illustrating the frequency characteristics of the longitudinally coupled resonator type surface acoustic wave filter of the first preferred embodiment and a conventional example prepared for comparison.
FIG. 3 shows a schematic plan view illustrating the electrode structure of the conventional longitudinally coupled resonator type surface acoustic wave filter prepared for comparison.
FIG. 4 shows a graph illustrating the frequency characteristics of longitudinally coupled resonator type surface acoustic wave filter units of both the first preferred embodiment and the conventional example prepared for comparison.
FIG. 5A and FIG. 5B show graphs for illustrating the principle of the longitudinally coupled resonator type surface acoustic wave filter of the first preferred embodiment, FIG. 5A showing the positions of resonance modes and FIG. 5B showing phase characteristics.
FIG. 6 shows a graph for illustrating the positions of resonance modes obtained when two pieces of the longitudinally coupled resonator type surface acoustic wave filter units are connected in parallel to each other in the longitudinally coupled resonator type surface acoustic wave filter according to the first preferred embodiment.
FIG. 7 shows a schematic plan view for illustrating a longitudinally coupled resonator type surface acoustic wave filter according to a modified example of the first preferred embodiment.
FIG. 8 shows a schematic plan view for illustrating a longitudinally coupled resonator type surface acoustic wave filter according to another modified example of the first preferred embodiment.
FIG. 9 shows a schematic plan view for illustrating a longitudinally coupled resonator type surface acoustic wave filter according to another modified example of the first preferred embodiment.
FIG. 10 shows a schematic plan view for illustrating the electrode structure of a conventional longitudinally coupled resonator type surface acoustic wave filter.
FIG. 11 shows a schematic plan view for illustrating a longitudinally coupled resonator type surface acoustic wave filter according to a second preferred embodiment.
FIG. 12 shows a schematic plan view for illustrating a longitudinally coupled resonator type surface acoustic wave filter according to a third preferred embodiment.
FIG. 13 shows a graph illustrating the frequency characteristics of the longitudinally coupled resonator type surface acoustic wave filter according to the third preferred embodiment and the frequency characteristics of the longitudinally coupled resonator type surface acoustic wave filter shown in FIG. 14, which is prepared for a comparison.
FIG. 14 shows a schematic plan view for illustrating the longitudinally coupled resonator type surface acoustic wave filter prepared to make the comparison with the third preferred embodiment.
FIG. 15 shows a schematic block diagram for illustrating an example of a communication apparatus formed by using the surface acoustic wave filter according to a preferred embodiment of the present invention.
FIG. 16 shows a schematic block diagram for illustrating another example of the communication apparatus formed by using the surface acoustic wave filter according to a preferred embodiment of the present invention.
FIG. 17 shows a schematic plan view for illustrating a longitudinally coupled resonator type surface acoustic wave filter according to a fourth preferred embodiment.
FIG. 18 shows a schematic plan view for illustrating one longitudinally coupled resonator type surface acoustic wave filter unit in the longitudinally coupled resonator type surface acoustic wave filter according to the fourth preferred embodiment.
FIG. 19 shows a schematic plan view for illustrating a longitudinally coupled resonator type surface acoustic wave filter prepared for comparison to preferred embodiments of the present invention.
FIG. 20 shows a graph illustrating the frequency characteristics of the longitudinally coupled resonator type surface acoustic wave filters of both the fourth preferred embodiment and the conventional example shown in FIG. 19.
FIG. 21 shows a schematic plan view for illustrating another conventional longitudinally coupled resonator type surface acoustic wave filter prepared for comparison to preferred embodiments of the present invention.
FIG. 22 shows a graph illustrating the frequency characteristics of the longitudinally coupled resonator type surface acoustic wave filter of the fourth preferred embodiment and the conventional example shown in FIG. 21.
FIG. 23 shows a graph illustrating the frequency characteristics of the longitudinally coupled resonator type surface acoustic wave filter of the fourth preferred embodiment and the longitudinally coupled resonator type surface acoustic wave filter shown in FIG. 21, which is modified by the method described in the conventional art.
FIG. 24 shows a schematic plan view of a modified example of the fourth preferred embodiment, illustrating a longitudinally coupled resonator type surface acoustic wave filter including central IDTs that have an odd number of electrode fingers.
FIG. 25 shows a graph of the frequency characteristics of both the fourth preferred embodiment and the longitudinally coupled resonator type surface acoustic wave filter as the modified example shown in FIG. 24.
FIG. 26 shows a schematic plan view of a longitudinally coupled resonator type surface acoustic wave filter according to a modified example of the first preferred embodiment.
FIG. 27 shows a graph of the frequency characteristics of a longitudinally coupled resonator type surface acoustic wave filter according to a modified example of the first preferred embodiment shown in FIG. 26.
FIG. 28 shows a schematic plan view of a longitudinally coupled resonator type surface acoustic wave filter according to another modified example of the first preferred embodiment.
FIG. 29 shows a graph of the frequency characteristics of a longitudinally coupled resonator type surface acoustic wave filter according to another modified example of the first preferred embodiment shown in FIG. 26.